On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal
at 60 kHz. One of the design goals is to keep the power consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing. Any
suggestions?


I agree with Jim. We need many more specifics to provide a meaningful
answer. There are a lot of micropower opamps out there now, but the
devil is in the details.

I've only found one detail that is giving me the devil. That is the
bandwidth. The signal is 60 kHz. I can't think of any other issues I
would have with any amp capable of amplifying this signal with a low
power level. What more info do you feel is needed? Can you ask
questions? Better yet, just point me to any amp that will meet my two
stated requirements!


The other posts you made had the info - things like impedance and gain
are important, as is frequency of operation (but we already know that).

A couple of things to consider, however. The higher the impedance, the
more susceptible it will be to ambient noise pickup. You're starting
with a very small signal and may need to add shielding to limit external
noise.

The other problem is you're asking for low impedance output. Low
impedance limits noise pickup, but increases current drain. So how low
of an impedance do you want?

I don't follow on this. How does a low output impedance drive the
current drain?


There are op amps with very high (in the gigaohm range) input impedance
and pretty low quiescent current drain. How much it draws during use
will be greatly dependent on the output current required, which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look through some of
my data sheets on op amps to see what I can find.

Thanks.

--

Rick

In article , rickman wrote:

I don't follow on this. How does a low output impedance drive the
current drain?

Well, the current to pull up the output against the low impedance has
to come from somewhere! It has to accounted for as part of the budget.

Consider the current used only by the amp, not the load.

You might want to experiment with using a one- or two-stage
common-drain amplifier using "electrometer grade" JFETs such as the
2N4117-2N4119 family (obsolete, but still available if you hunt around
a bit). They have extremely low gate leakage (i.e. very high input
impedance). Even with 0 volts on the gate (relative to source) they
have a low Idss, and of course you can add a resistor between source
and ground and reduce the standing current as low as you want.

On 11/6/2014 2:02 PM, David Platt wrote:
In article , rickman wrote:

I don't follow on this. How does a low output impedance drive the
current drain?

Well, the current to pull up the output against the low impedance has
to come from somewhere! It has to accounted for as part of the budget.

I never said the load is low impedance. I said the output should be
low impedance.


Consider the current used only by the amp, not the load.

You might want to experiment with using a one- or two-stage
common-drain amplifier using "electrometer grade" JFETs such as the
2N4117-2N4119 family (obsolete, but still available if you hunt around
a bit). They have extremely low gate leakage (i.e. very high input
impedance). Even with 0 volts on the gate (relative to source) they
have a low Idss, and of course you can add a resistor between source
and ground and reduce the standing current as low as you want.

I'll take a look. The input impedance does not need to be *huge*. My
concern is the drain current. This needs to be operated in an analog
range rather than digital and provide gain.

--

Rick

On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal
at 60 kHz. One of the design goals is to keep the power
consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing. Any
suggestions?


I agree with Jim. We need many more specifics to provide a meaningful
answer. There are a lot of micropower opamps out there now, but the
devil is in the details.

I've only found one detail that is giving me the devil. That is the
bandwidth. The signal is 60 kHz. I can't think of any other issues I
would have with any amp capable of amplifying this signal with a low
power level. What more info do you feel is needed? Can you ask
questions? Better yet, just point me to any amp that will meet my two
stated requirements!


The other posts you made had the info - things like impedance and gain
are important, as is frequency of operation (but we already know that).

A couple of things to consider, however. The higher the impedance, the
more susceptible it will be to ambient noise pickup. You're starting
with a very small signal and may need to add shielding to limit external
noise.

The other problem is you're asking for low impedance output. Low
impedance limits noise pickup, but increases current drain. So how low
of an impedance do you want?

I don't follow on this. How does a low output impedance drive the
current drain?


There are op amps with very high (in the gigaohm range) input impedance
and pretty low quiescent current drain. How much it draws during use
will be greatly dependent on the output current required, which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look through some of
my data sheets on op amps to see what I can find.

Thanks.


Total current is not just dependent on output current; it also is
affected by the design of the chip. Op amps are not just single
transistor devices; a lower output impedance also means more current to
drive the output stage, which affects other components. So even if you
have a high impedance load, the lower the output impedance of the op amp
(i.e. the more current it can source/sink at a specific supply voltage),
the more overall current the op amp will draw.

With that said, I did some looking around (sorry for not getting back to
you quicker - yesterday was pretty busy). Depending on your needs,
there are hundreds you can choose from. I might recommend you check out
http://www.mouser.com/Semiconductors...mps/_/N-6j73m/
.. You can pick and choose the parameters you want. Another one I've
used is http://www.newark.com/operational-amplifiers.

Between the two I found several hundred possibilities, but you know the
details of what you want better than I do, so rather than guess at what
you might want, I think this would be better. It should give you a start.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 11/7/2014 10:49 AM, Jerry Stuckle wrote:
On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal
at 60 kHz. One of the design goals is to keep the power
consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing. Any
suggestions?


I agree with Jim. We need many more specifics to provide a meaningful
answer. There are a lot of micropower opamps out there now, but the
devil is in the details.

I've only found one detail that is giving me the devil. That is the
bandwidth. The signal is 60 kHz. I can't think of any other issues I
would have with any amp capable of amplifying this signal with a low
power level. What more info do you feel is needed? Can you ask
questions? Better yet, just point me to any amp that will meet my two
stated requirements!


The other posts you made had the info - things like impedance and gain
are important, as is frequency of operation (but we already know that).

A couple of things to consider, however. The higher the impedance, the
more susceptible it will be to ambient noise pickup. You're starting
with a very small signal and may need to add shielding to limit external
noise.

The other problem is you're asking for low impedance output. Low
impedance limits noise pickup, but increases current drain. So how low
of an impedance do you want?

I don't follow on this. How does a low output impedance drive the
current drain?


There are op amps with very high (in the gigaohm range) input impedance
and pretty low quiescent current drain. How much it draws during use
will be greatly dependent on the output current required, which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look through some of
my data sheets on op amps to see what I can find.

Thanks.


Total current is not just dependent on output current; it also is
affected by the design of the chip. Op amps are not just single
transistor devices; a lower output impedance also means more current to
drive the output stage, which affects other components. So even if you
have a high impedance load, the lower the output impedance of the op amp
(i.e. the more current it can source/sink at a specific supply voltage),
the more overall current the op amp will draw.

With that said, I did some looking around (sorry for not getting back to
you quicker - yesterday was pretty busy). Depending on your needs,
there are hundreds you can choose from. I might recommend you check out
http://www.mouser.com/Semiconductors...mps/_/N-6j73m/
. You can pick and choose the parameters you want. Another one I've
used is http://www.newark.com/operational-amplifiers.

Between the two I found several hundred possibilities, but you know the
details of what you want better than I do, so rather than guess at what
you might want, I think this would be better. It should give you a start.

I have done this before and found nothing. But I did it again at both
Mouser and Digikey and found several. One listed by Mouser looked
especially good only to find rather than 0.75 uA of supply current, it
had 0.75 mA of supply current. lol

But then the next part, same thing... another one... and another... one
part I'm not sure what to make of it. The selection table shows supply
current of 0.034 mA and the data sheet shows 25 A! Yes, that's right,
the data sheet shows between 25 and 300 Amps for typical supply
current!!! I would contact TI about this obvious typo, but this part is
not suitable because of the GBW which is also incorrect in the selection
table.

Same thing at Digikey, everything in the selection table that meets
these two requirements is a mistake.

--

Rick

On 11/7/2014 1:02 PM, rickman wrote:
On 11/7/2014 10:49 AM, Jerry Stuckle wrote:
On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth
signal
at 60 kHz. One of the design goals is to keep the power
consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing.
Any
suggestions?


I agree with Jim. We need many more specifics to provide a
meaningful
answer. There are a lot of micropower opamps out there now, but the
devil is in the details.

I've only found one detail that is giving me the devil. That is the
bandwidth. The signal is 60 kHz. I can't think of any other issues I
would have with any amp capable of amplifying this signal with a low
power level. What more info do you feel is needed? Can you ask
questions? Better yet, just point me to any amp that will meet my two
stated requirements!


The other posts you made had the info - things like impedance and gain
are important, as is frequency of operation (but we already know that).

A couple of things to consider, however. The higher the impedance, the
more susceptible it will be to ambient noise pickup. You're starting
with a very small signal and may need to add shielding to limit
external
noise.

The other problem is you're asking for low impedance output. Low
impedance limits noise pickup, but increases current drain. So how low
of an impedance do you want?

I don't follow on this. How does a low output impedance drive the
current drain?


There are op amps with very high (in the gigaohm range) input impedance
and pretty low quiescent current drain. How much it draws during use
will be greatly dependent on the output current required, which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look through some of
my data sheets on op amps to see what I can find.

Thanks.


Total current is not just dependent on output current; it also is
affected by the design of the chip. Op amps are not just single
transistor devices; a lower output impedance also means more current to
drive the output stage, which affects other components. So even if you
have a high impedance load, the lower the output impedance of the op amp
(i.e. the more current it can source/sink at a specific supply voltage),
the more overall current the op amp will draw.

With that said, I did some looking around (sorry for not getting back to
you quicker - yesterday was pretty busy). Depending on your needs,
there are hundreds you can choose from. I might recommend you check out
http://www.mouser.com/Semiconductors...mps/_/N-6j73m/

. You can pick and choose the parameters you want. Another one I've
used is http://www.newark.com/operational-amplifiers.

Between the two I found several hundred possibilities, but you know the
details of what you want better than I do, so rather than guess at what
you might want, I think this would be better. It should give you a
start.

I have done this before and found nothing. But I did it again at both
Mouser and Digikey and found several. One listed by Mouser looked
especially good only to find rather than 0.75 uA of supply current, it
had 0.75 mA of supply current. lol

But then the next part, same thing... another one... and another... one
part I'm not sure what to make of it. The selection table shows supply
current of 0.034 mA and the data sheet shows 25 A! Yes, that's right,
the data sheet shows between 25 and 300 Amps for typical supply
current!!! I would contact TI about this obvious typo, but this part is
not suitable because of the GBW which is also incorrect in the selection
table.

Same thing at Digikey, everything in the selection table that meets
these two requirements is a mistake.


A couple of things.

First of all, I've found minor errors in the listings at Mouser (I don't
use Digikey much), but never real glaring errors. And this is th first
time I've seen a TI datasheet that far off. Looks like someone dropped
a decimal point . However, I've found Mouser is interested in
correcting errors; they are input by humans, after all, at some point in
time, and errors do creep in.

Secondly, the current shown is going to be max current, which will
depend on the output impedance (and the amount that has to be
sourced/sunk). It's not going to pull this all the time; I would expect
your actual current draw to be much less since you're 1) going into a
high impedance load and 2) not going from rail to rail.

Also, if you use a bipolar supply, then current drain should be less
because you'll be operating near ground, instead of the midpoint of a
single supply voltage (where the output would be at 1/2 Vcc). Some of
these are quite low voltage, and I would think a couple of the larger
lithium coin batteries should last quite a while.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 11/7/2014 1:17 PM, Jerry Stuckle wrote:
On 11/7/2014 1:02 PM, rickman wrote:
On 11/7/2014 10:49 AM, Jerry Stuckle wrote:
On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth
signal
at 60 kHz. One of the design goals is to keep the power
consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing.
Any
suggestions?


I agree with Jim. We need many more specifics to provide a
meaningful
answer. There are a lot of micropower opamps out there now, but the
devil is in the details.

I've only found one detail that is giving me the devil. That is the
bandwidth. The signal is 60 kHz. I can't think of any other issues I
would have with any amp capable of amplifying this signal with a low
power level. What more info do you feel is needed? Can you ask
questions? Better yet, just point me to any amp that will meet my two
stated requirements!


The other posts you made had the info - things like impedance and gain
are important, as is frequency of operation (but we already know that).

A couple of things to consider, however. The higher the impedance, the
more susceptible it will be to ambient noise pickup. You're starting
with a very small signal and may need to add shielding to limit
external
noise.

The other problem is you're asking for low impedance output. Low
impedance limits noise pickup, but increases current drain. So how low
of an impedance do you want?

I don't follow on this. How does a low output impedance drive the
current drain?


There are op amps with very high (in the gigaohm range) input impedance
and pretty low quiescent current drain. How much it draws during use
will be greatly dependent on the output current required, which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look through some of
my data sheets on op amps to see what I can find.

Thanks.


Total current is not just dependent on output current; it also is
affected by the design of the chip. Op amps are not just single
transistor devices; a lower output impedance also means more current to
drive the output stage, which affects other components. So even if you
have a high impedance load, the lower the output impedance of the op amp
(i.e. the more current it can source/sink at a specific supply voltage),
the more overall current the op amp will draw.

With that said, I did some looking around (sorry for not getting back to
you quicker - yesterday was pretty busy). Depending on your needs,
there are hundreds you can choose from. I might recommend you check out
http://www.mouser.com/Semiconductors...mps/_/N-6j73m/

. You can pick and choose the parameters you want. Another one I've
used is http://www.newark.com/operational-amplifiers.

Between the two I found several hundred possibilities, but you know the
details of what you want better than I do, so rather than guess at what
you might want, I think this would be better. It should give you a
start.

I have done this before and found nothing. But I did it again at both
Mouser and Digikey and found several. One listed by Mouser looked
especially good only to find rather than 0.75 uA of supply current, it
had 0.75 mA of supply current. lol

But then the next part, same thing... another one... and another... one
part I'm not sure what to make of it. The selection table shows supply
current of 0.034 mA and the data sheet shows 25 A! Yes, that's right,
the data sheet shows between 25 and 300 Amps for typical supply
current!!! I would contact TI about this obvious typo, but this part is
not suitable because of the GBW which is also incorrect in the selection
table.

Same thing at Digikey, everything in the selection table that meets
these two requirements is a mistake.


A couple of things.

First of all, I've found minor errors in the listings at Mouser (I don't
use Digikey much), but never real glaring errors. And this is th first
time I've seen a TI datasheet that far off. Looks like someone dropped
a decimal point . However, I've found Mouser is interested in
correcting errors; they are input by humans, after all, at some point in
time, and errors do creep in.

Yes, when you list millions of parts there will be errors. I have
written digikey many times about listing errors and they always thank
me. I'm sure Mouser is no different.


Secondly, the current shown is going to be max current, which will
depend on the output impedance (and the amount that has to be
sourced/sunk). It's not going to pull this all the time; I would expect
your actual current draw to be much less since you're 1) going into a
high impedance load and 2) not going from rail to rail.

I find the opposite. The current listed is under specified conditions
which usually *do not* include output drive. In fact, it usually listed
as a quiescent current.


Also, if you use a bipolar supply, then current drain should be less
because you'll be operating near ground, instead of the midpoint of a
single supply voltage (where the output would be at 1/2 Vcc). Some of
these are quite low voltage, and I would think a couple of the larger
lithium coin batteries should last quite a while.

Not sure how the ground level would affect the bias currents. When the
supply voltage is lowered the GBW lowers as well.

--

Rick

On 11/7/2014 1:26 PM, rickman wrote:
On 11/7/2014 1:17 PM, Jerry Stuckle wrote:
On 11/7/2014 1:02 PM, rickman wrote:
On 11/7/2014 10:49 AM, Jerry Stuckle wrote:
On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth
signal
at 60 kHz. One of the design goals is to keep the power
consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing.
Any
suggestions?


I agree with Jim. We need many more specifics to provide a
meaningful
answer. There are a lot of micropower opamps out there now, but
the
devil is in the details.

I've only found one detail that is giving me the devil. That is the
bandwidth. The signal is 60 kHz. I can't think of any other
issues I
would have with any amp capable of amplifying this signal with a low
power level. What more info do you feel is needed? Can you ask
questions? Better yet, just point me to any amp that will meet
my two
stated requirements!


The other posts you made had the info - things like impedance and
gain
are important, as is frequency of operation (but we already know
that).

A couple of things to consider, however. The higher the
impedance, the
more susceptible it will be to ambient noise pickup. You're starting
with a very small signal and may need to add shielding to limit
external
noise.

The other problem is you're asking for low impedance output. Low
impedance limits noise pickup, but increases current drain. So
how low
of an impedance do you want?

I don't follow on this. How does a low output impedance drive the
current drain?


There are op amps with very high (in the gigaohm range) input
impedance
and pretty low quiescent current drain. How much it draws during use
will be greatly dependent on the output current required, which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look through
some of
my data sheets on op amps to see what I can find.

Thanks.


Total current is not just dependent on output current; it also is
affected by the design of the chip. Op amps are not just single
transistor devices; a lower output impedance also means more current to
drive the output stage, which affects other components. So even if you
have a high impedance load, the lower the output impedance of the op
amp
(i.e. the more current it can source/sink at a specific supply
voltage),
the more overall current the op amp will draw.

With that said, I did some looking around (sorry for not getting
back to
you quicker - yesterday was pretty busy). Depending on your needs,
there are hundreds you can choose from. I might recommend you check
out
http://www.mouser.com/Semiconductors...mps/_/N-6j73m/


. You can pick and choose the parameters you want. Another one I've
used is http://www.newark.com/operational-amplifiers.

Between the two I found several hundred possibilities, but you know the
details of what you want better than I do, so rather than guess at what
you might want, I think this would be better. It should give you a
start.

I have done this before and found nothing. But I did it again at both
Mouser and Digikey and found several. One listed by Mouser looked
especially good only to find rather than 0.75 uA of supply current, it
had 0.75 mA of supply current. lol

But then the next part, same thing... another one... and another... one
part I'm not sure what to make of it. The selection table shows supply
current of 0.034 mA and the data sheet shows 25 A! Yes, that's right,
the data sheet shows between 25 and 300 Amps for typical supply
current!!! I would contact TI about this obvious typo, but this part is
not suitable because of the GBW which is also incorrect in the selection
table.

Same thing at Digikey, everything in the selection table that meets
these two requirements is a mistake.


A couple of things.

First of all, I've found minor errors in the listings at Mouser (I don't
use Digikey much), but never real glaring errors. And this is th first
time I've seen a TI datasheet that far off. Looks like someone dropped
a decimal point . However, I've found Mouser is interested in
correcting errors; they are input by humans, after all, at some point in
time, and errors do creep in.

Yes, when you list millions of parts there will be errors. I have
written digikey many times about listing errors and they always thank
me. I'm sure Mouser is no different.


Secondly, the current shown is going to be max current, which will
depend on the output impedance (and the amount that has to be
sourced/sunk). It's not going to pull this all the time; I would expect
your actual current draw to be much less since you're 1) going into a
high impedance load and 2) not going from rail to rail.

I find the opposite. The current listed is under specified conditions
which usually *do not* include output drive. In fact, it usually listed
as a quiescent current.


Well, yes and no. Op amps typically sink more than they source, and the
sink current does not come from the chip. Source current at the output
is supplied by the chip, of course.

And I've found a wide difference between how op amp specs are listed;
some show quiescent current, some show average current under typical
operating conditions. Some even show maximum current which can be
drawn. So I'll retract that statement above. Wasn't thinking clearly.


Also, if you use a bipolar supply, then current drain should be less
because you'll be operating near ground, instead of the midpoint of a
single supply voltage (where the output would be at 1/2 Vcc). Some of
these are quite low voltage, and I would think a couple of the larger
lithium coin batteries should last quite a while.

Not sure how the ground level would affect the bias currents. When the
supply voltage is lowered the GBW lowers as well.


If the output is at ground level, no current will be pulled from either
rail (at the output). Shifting above or below that will draw a little
current, reference zero. However, if you're running a single ended
supply, your output will be at 1/2 Vcc, and will always be pulling some
current to maintain that level. The signal will change that slightly,
increasing or decreasing. But unless you have a square wave with a 50%
duty cycle, you'll end up needing more current from the single ended
supply.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 11/7/2014 1:53 PM, Jerry Stuckle wrote:
On 11/7/2014 1:26 PM, rickman wrote:
On 11/7/2014 1:17 PM, Jerry Stuckle wrote:
On 11/7/2014 1:02 PM, rickman wrote:
On 11/7/2014 10:49 AM, Jerry Stuckle wrote:
On 11/6/2014 11:45 AM, rickman wrote:
On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth
signal
at 60 kHz. One of the design goals is to keep the power
consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing.
Any
suggestions?


I agree with Jim. We need many more specifics to provide a
meaningful
answer. There are a lot of micropower opamps out there now, but
the
devil is in the details.

I've only found one detail that is giving me the devil. That is the
bandwidth. The signal is 60 kHz. I can't think of any other
issues I
would have with any amp capable of amplifying this signal with a low
power level. What more info do you feel is needed? Can you ask
questions? Better yet, just point me to any amp that will meet
my two
stated requirements!


The other posts you made had the info - things like impedance and
gain
are important, as is frequency of operation (but we already know
that).

A couple of things to consider, however. The higher the
impedance, the
more susceptible it will be to ambient noise pickup. You're starting
with a very small signal and may need to add shielding to limit
external
noise.

The other problem is you're asking for low impedance output. Low
impedance limits noise pickup, but increases current drain. So
how low
of an impedance do you want?

I don't follow on this. How does a low output impedance drive the
current drain?


There are op amps with very high (in the gigaohm range) input
impedance
and pretty low quiescent current drain. How much it draws during use
will be greatly dependent on the output current required, which
obviously depends on output voltage and impedance.

Consider the current used only by the amp, not the load.


I don't have time right now, but later today I'll look through
some of
my data sheets on op amps to see what I can find.

Thanks.


Total current is not just dependent on output current; it also is
affected by the design of the chip. Op amps are not just single
transistor devices; a lower output impedance also means more current to
drive the output stage, which affects other components. So even if you
have a high impedance load, the lower the output impedance of the op
amp
(i.e. the more current it can source/sink at a specific supply
voltage),
the more overall current the op amp will draw.

With that said, I did some looking around (sorry for not getting
back to
you quicker - yesterday was pretty busy). Depending on your needs,
there are hundreds you can choose from. I might recommend you check
out
http://www.mouser.com/Semiconductors...mps/_/N-6j73m/


. You can pick and choose the parameters you want. Another one I've
used is http://www.newark.com/operational-amplifiers.

Between the two I found several hundred possibilities, but you know the
details of what you want better than I do, so rather than guess at what
you might want, I think this would be better. It should give you a
start.

I have done this before and found nothing. But I did it again at both
Mouser and Digikey and found several. One listed by Mouser looked
especially good only to find rather than 0.75 uA of supply current, it
had 0.75 mA of supply current. lol

But then the next part, same thing... another one... and another... one
part I'm not sure what to make of it. The selection table shows supply
current of 0.034 mA and the data sheet shows 25 A! Yes, that's right,
the data sheet shows between 25 and 300 Amps for typical supply
current!!! I would contact TI about this obvious typo, but this part is
not suitable because of the GBW which is also incorrect in the selection
table.

Same thing at Digikey, everything in the selection table that meets
these two requirements is a mistake.


A couple of things.

First of all, I've found minor errors in the listings at Mouser (I don't
use Digikey much), but never real glaring errors. And this is th first
time I've seen a TI datasheet that far off. Looks like someone dropped
a decimal point . However, I've found Mouser is interested in
correcting errors; they are input by humans, after all, at some point in
time, and errors do creep in.

Yes, when you list millions of parts there will be errors. I have
written digikey many times about listing errors and they always thank
me. I'm sure Mouser is no different.


Secondly, the current shown is going to be max current, which will
depend on the output impedance (and the amount that has to be
sourced/sunk). It's not going to pull this all the time; I would expect
your actual current draw to be much less since you're 1) going into a
high impedance load and 2) not going from rail to rail.

I find the opposite. The current listed is under specified conditions
which usually *do not* include output drive. In fact, it usually listed
as a quiescent current.


Well, yes and no. Op amps typically sink more than they source, and the
sink current does not come from the chip. Source current at the output
is supplied by the chip, of course.

And I've found a wide difference between how op amp specs are listed;
some show quiescent current, some show average current under typical
operating conditions. Some even show maximum current which can be
drawn. So I'll retract that statement above. Wasn't thinking clearly.


Also, if you use a bipolar supply, then current drain should be less
because you'll be operating near ground, instead of the midpoint of a
single supply voltage (where the output would be at 1/2 Vcc). Some of
these are quite low voltage, and I would think a couple of the larger
lithium coin batteries should last quite a while.

Not sure how the ground level would affect the bias currents. When the
supply voltage is lowered the GBW lowers as well.


If the output is at ground level, no current will be pulled from either
rail (at the output). Shifting above or below that will draw a little
current, reference zero. However, if you're running a single ended
supply, your output will be at 1/2 Vcc, and will always be pulling some
current to maintain that level. The signal will change that slightly,
increasing or decreasing. But unless you have a square wave with a 50%
duty cycle, you'll end up needing more current from the single ended
supply.

What you are saying is only true if your load is ground connected. The
load for this circuit will be a voltage source through a high impedance.
The input is differential and to make it as sensitive as possible a
bias will be applied to one input sufficient to offset the input bias
voltage. So in reality the load will be biased to approx 1/2 Vcc.

The real point is that this is *LOAD* current, not amplifier current and
is independent of the amplifier and so considered separately since
selection of the amplifier has no impact on it.

--

Rick

On Thu, 6 Nov 2014, rickman wrote:

On 11/6/2014 10:04 AM, Jerry Stuckle wrote:
On 11/5/2014 1:29 PM, rickman wrote:
On 11/4/2014 9:42 PM, Jerry Stuckle wrote:
On 11/4/2014 6:29 PM, rickman wrote:
I am working on a project for receiving a very narrow bandwidth signal
at 60 kHz. One of the design goals is to keep the power consumption to
an absolute minimum. I'm trying to figure out how to run a
pre-amplifier on less than 100 uW. So far I have found nothing. Any
suggestions?


I agree with Jim. We need many more specifics to provide a meaningful
answer. There are a lot of micropower opamps out there now, but the
devil is in the details.

I've only found one detail that is giving me the devil. That is the
bandwidth. The signal is 60 kHz. I can't think of any other issues I
would have with any amp capable of amplifying this signal with a low
power level. What more info do you feel is needed? Can you ask
questions? Better yet, just point me to any amp that will meet my two
stated requirements!


The other posts you made had the info - things like impedance and gain
are important, as is frequency of operation (but we already know that).

A couple of things to consider, however. The higher the impedance, the
more susceptible it will be to ambient noise pickup. You're starting
with a very small signal and may need to add shielding to limit external
noise.

The other problem is you're asking for low impedance output. Low
impedance limits noise pickup, but increases current drain. So how low
of an impedance do you want?

I don't follow on this. How does a low output impedance drive the current
drain?

If you use a large resister in the collector, you'll get high impedance
output. But load it down with a low impedance, and there won't be a proper transfer of the
signal.

So you use a low value collector resistor, current goes up because it
pushes more current through the device, but you get your lower impedance.

I thought generally people wanted more current into preamps, because that
helped them in handling strong signals. So take a look at WWVB
preamps/receivers from the seventies. Certainly they'd be using bipolar
transistors, but one might think they might be reasonably low current. On
the other hand, I can't remember why you need low current for this, and
something like a WWVB receiver usually doesn't need to fuss about being
extra low current. So I suspect those projects never tried to be
ultra-low current.

My Casio Waveceptor watch does, but I have no idea what kind of circuitry
is in there, and even if I opened the watch, I bet it would be difficult
to trace.

On the other hand, I have a Radio Shack "atomic clock" that runs for years
on one AAA or AA battery, so someone figured out how to receive WWVB with
low current and low voltage. But then, the WWVB front end is likely a
module, which is another way to solve the problem, just buy a module, or
strip one out of an existing clock. But again, I can't remember why you
are needing this, so I suspect there's some reason why these options
aren't being used.

Is a preamp really going to be low current compared to the later
circuitry's needs? Once you add the rest, maybe it's not worth pursuing
ultra-low current for the preamp.

Michael